Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1-xGax (SySe1-y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Among the family of compounds, best efficiencies have been obtained for those containing both Ga and In, with a Ga amount in the 15-25%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. Therefore, in summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in FIG. 1. The device 10 is fabricated on a substrate 11, such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. The absorber film 12, which comprises a material in the family of Cu(In,Ga,Al)(S,Se,Te)2, is grown over a conductive layer 13 or a contact layer, which is previously deposited on the substrate 11 and which acts as the electrical ohmic contact to the device. The stack of the substrate 11 and the conductive layer 13 forms a base 20. The most commonly used contact layer or conductive layer in the solar cell structure of FIG. 1 is Molybdenum (Mo). If the substrate itself is a properly selected conductive material such as a Mo foil, it is possible not to use a conductive layer 13, since the substrate 11 may then be used as the ohmic contact to the device. The conductive layer 13 may also act as a diffusion barrier in case the metallic foil is reactive. For example, metallic foils comprising materials such as Al, Ti, Ni, Cu may be used as substrates provided a barrier such as a Mo layer is deposited on them protecting them from Se or S vapors. The barrier is often deposited on both sides of the foil to protect it well. After the absorber film 12 is grown, a transparent layer 14 such as a CdS, ZnO or CdS/ZnO stack is formed on the absorber film. Radiation 15 enters the device through the transparent layer 14. Metallic grids (not shown) may also be deposited over the transparent layer 14 to reduce the effective series resistance of the device. The preferred electrical type of the absorber film 12 is p-type, and the preferred electrical type of the transparent layer 14 is n-type. However, an n-type absorber and a p-type window layer can also be utilized.
The preferred device structure of FIG. 1 is called a “substrate-type” structure. A “superstrate-type” structure can also be constructed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te)2 absorber film, and finally forming an ohmic contact to the device by a conductive layer. In this superstrate structure light enters the device from the transparent superstrate side. A variety of materials, deposited by a variety of methods, can be used to provide the various layers of the device shown in FIG. 1. It should be noted that although the chemical formula of copper indium gallium sulfo-selenide is often written as Cu(In,Ga)(S,Se)2, a more accurate formula for the compound is Cu(In,Ga)(S,Se)k, where k is typically close to 2 but may not be exactly 2. For simplicity the value of k will be assumed as 2. It should be further noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se)2 means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1.
The first technique that yielded high-quality Cu(In,Ga)Se2 films for solar cell fabrication was co-evaporation of Cu, In, Ga and Se onto a heated substrate in a vacuum chamber. This is an approach with low materials utilization and high cost of equipment.
Another technique for growing Cu(In,Ga)(S,Se)2 type compound thin films for solar cell applications is a two-stage process where metallic components of the Cu(In,Ga)(S,Se)2 material are first deposited onto a substrate, and then reacted with S and/or Se in a high temperature annealing process. For example, for CuInSe2 growth, thin layers of Cu and In are first deposited on a substrate and then this stacked precursor layer is reacted with Se at elevated temperature. If the reaction atmosphere also contains sulfur, then a CuIn(S,Se)2 layer can be grown. Addition of Ga in the precursor layer, i.e. use of a Cu/In/Ga stacked film precursor, allows the growth of a Cu(In,Ga)(S,Se)2 absorber.
Sputtering and evaporation techniques have been used in prior art approaches to deposit the layers containing the Group IB and Group IIIA components of the precursor stacks. In the case of CuInSe2 growth, for example, Cu and In layers were sequentially sputter-deposited on a substrate and then the stacked film was heated in the presence of gas containing Se at elevated temperature for times typically longer than about 30 minutes, as described in U.S. Pat. No. 4,798,660. More recently U.S. Pat. No. 6,048,442 disclosed a method comprising sputter-depositing a stacked precursor film comprising a Cu—Ga alloy layer(s) and an In layer to form a Cu—Ga/In stack on a metallic back electrode layer and then reacting this precursor stack film with one of Se and S to form the absorber layer. U.S. Pat. No. 6,092,669 described sputtering-based equipment for producing such absorber layers.
One prior art method described in U.S. Pat. No. 4,581,108 utilizes a low cost electrodeposition approach for metallic precursor preparation. In this method a Cu layer is first electrodeposited on a substrate covered with Mo. This is then followed by electrodeposition of an In layer and heating of the deposited Cu/In stack in a reactive atmosphere containing Se to obtain CIS.
Prior investigations on possible dopants for Group IBIIIAVIA compound layers have shown that alkali metals, such as Na, K, and Li, affect the structural and electrical properties of such layers. Especially, inclusion of Na in CIGS layers was shown to be beneficial for increasing the conversion efficiencies of solar cells fabricated on such layers provided that its concentration is well controlled. Inclusion of Na into CIGS layers was achieved by various ways. For example, Na was diffused into the CIGS layer from the substrate if the CIGS film was grown on a Mo contact layer deposited on a Na-containing soda-lime glass substrate. This approach, however, is hard to control and causes non-uniformities in the CIGS layers depending on how much Na diffuses from the substrate through the Mo contact layer. Therefore the amount of Na doping is a strong function of the nature of the Mo layer such as its grain size, crystalline structure, chemical composition, thickness, etc. In another approach (see for example, U.S. Pat. No. 5,626,688), a diffusion barrier is deposited on the soda-lime glass substrate to stop possible Na diffusion from the substrate into the absorber layer. A Mo contact film is then deposited on the diffusion barrier. An interfacial layer comprising Na is formed on the Mo surface. The CIGS film is then grown over the Na containing interfacial layer.
During the growth period, Na from the interfacial layer gets included into the CIGS layer and dopes it. The most commonly used interfacial layer material is NaF, which is evaporated on the Mo surface before the deposition of the CIGS layer by the co-evaporation technique (see, for example, Granath et al., Solar Energy Materials and Solar Cells, vol: 60, p: 279 (2000)). U.S. Pat. No. 7,018,858 describes a method of fabricating a layer of CIGS wherein an alkali layer is formed on the back electrode (typically Mo) by dipping the back electrode in an aqueous solution containing alkali metals, drying the layer, forming a precursor layer on the alkali layer and heat treating the precursor in a selenium atmosphere. Another method of supplying Na to the growing CIGS layer is depositing a Na-doped Mo layer on the substrate, following this by deposition of an un-doped Mo layer and growing the CIGS film over the undoped Mo layer. In this case Na from the Na-doped Mo layer diffuses through the undoped Mo layer and enters the CIGS film during high temperature growth.
It should be noted that the methods described above include the alkali metal early in the process and the alkali metal, such as Na, participates in the overall reaction as Cu, In, Ga and Se, optionally S react with each other forming the CIGS(S) compound film on the base. For example, a stack may be formed in accordance with prior art approaches where a Na-containing layer may first be deposited on a base comprising a Mo layer coated on a substrate. The Na-containing layer may be deposited on the Mo surface and then this may be followed by the deposition of a metallic precursor comprising Cu, In and Ga. When this stack is heated up in presence of Se to form a CIGS layer, all elements, i.e. Cu, In, Ga, Se and Na participate in this reaction. Same is true when Cu, In, Ga and Se are co-evaporated on a heated substrate surface covered by a Mo layer and a Na-containing layer. In these approaches, participation of the alkali, such as Na, in the reaction, may yield Na-containing phases such as Na-selenide (and/or Na-sulfide if S is present) compounds as alloys between Na and any one of Cu, In and Ga. The Na amount that is actually useful for doping, then needs to be experimentally determined since some Na is used for other reactions. Formation of excess phases such as Na—Se and or S may also reduce availability of Se and/or S to the actual CIGS(S) compound formation and thus cause problems with repeatability of the process as well as materials utilization. Sodium also influences the morphology and grain size of CIGS-type materials. Rudmann et al. (Thin Solid Films, vol: 431-432, p: 37, 2003), for example, observed a reduction in grain size of the CIGS films when Na was available during the growth of the compound layers.
As the review above demonstrates, controlled doping of Group IBIIIAVIA compound layers with alkali metals improve their quality in terms of yielding higher efficiency solar cell devices, however, there is still need for alternative methods to introduce the dopants into the compound layers in an efficient manner that does not negatively impact their morphological and electrical properties.